Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes of tempering, bending, or the like. Heat treatment (HT) of coated articles typically requires use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and still more preferably of at least 620 degrees C. Such high temperatures (e.g., for 5-10 minutes or more) often cause coatings to break down and/or deteriorate or change in an unpredictable manner. Thus, it is desirable for coatings to be able to withstand such heat treatments (e.g., thermal tempering), if desired, in a predictable manner that does not significantly damage the coating.
In certain situations, designers of coated articles strive for a combination of desirable visible transmission, desirable color, low emissivity (or emittance), and low sheet resistance (Rs). Low-emissivity (low-E) and/or low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors.
U.S. Pat. No. 7,521,096, incorporated herein by reference, discloses a low-E coating which uses zinc oxide (ZnO) contact layers below the silver-based IR reflecting layers, and above the bottom silver (Ag) based IR reflecting layer uses a NiCrOx contact layer followed by a center tin oxide (SnO2) dielectric layer. The SnO2 in the coating of the '096 patent has been found to show micro crystallization and stress upon HT which causes rough interfaces between the SnO2, the ZnO and the Ag, which can lead to degradation of durability and affect transmitted color. The rough metal inclusive interfaces tend to initiate so called surface plasmons which result in selective absorption in a full layer stack especially in the lower visible spectral region (380-550 nm). This tends to cause one or more of unpredictable optical characteristics, less neutral color, lower visible transmission and/or lower LSG. Moreover, it has been found that the tin oxide (e.g., SnO2) goes through significant change in crystallization and stress upon heating which tends to degrade the structure of ZnO and hence the Ag growth thereon. Again, this can lead to poor Ag quality and/or poor thermal stability of the coating such as undesirably high ΔE* values.